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Summary of Significant Accounting Policies - Narrative (Details)
$ in Thousands
 12 Months Ended
Dec. 31, 2017
USD ($)
customer
Dec. 31, 2016
USD ($)
customer
Dec. 31, 2015
USD ($)
customer
Schedule of Accounting Policies [Line Items]      
Investments in certificates of deposit $ 2,900 $ 5,500  
Certificates of deposit interest rate, lower range 0.95%    
Certificates of deposit interest rate, upper range 1.10%    
Investments held to maturity $ 6,077 14,083  
Research and development costs 13,000 12,000 $ 7,500
Advertising expense 1,700 1,400 1,200
Shipping and handling fees 11,400 10,300 9,600
Payments to representatives $ 51,800 $ 55,000 $ 55,400
Extended product warranties, minimum length 6 months    
Extended product warranties, maximum length 10 years    
Foreign Sales Revenue, Net      
Schedule of Accounting Policies [Line Items]      
Concentration risk, percentage 4.00% 4.00% 4.00%
Sales Revenue, Net      
Schedule of Accounting Policies [Line Items]      
Number of major customers | customer 1 1 1
Accounting for More Than 5% of Accounts Receivable      
Schedule of Accounting Policies [Line Items]      
Number of major customers 0 0  
Concentration risk, percentage 5.00% 5.00%  
Minimum      
Schedule of Accounting Policies [Line Items]      
Certificates of deposit maturity dates 1 month    
Investments maturity date 1 month    
Maximum      
Schedule of Accounting Policies [Line Items]      
Certificates of deposit maturity dates 5 months    
Investments maturity date 4 months